The present invention concerns a method of producing a cladding tube for nuclear fuel for a nuclear boiling water reactor, which method comprises the following steps:
forming a tube which comprises an outer cylindrical component mainly containing zirconium and an inner cylindrical component metallurgically bonded to the outer component, wherein also the inner component at least mainly contains zirconium, wherein the material compositions of the inner component and the outer component are selected such that they differ from each other and such that the inner component has a lower recrystallization temperature than the outer component.
The invention also concerns a cladding tube, a use of a cladding tube as well as a fuel assembly for a nuclear boiling water reactor comprising such a cladding tube.
A method of the kind that is described in the first paragraph above is known from the patent document EP 0 674 800 B1. In this document also the background to the invention described therein is described. When a cladding tube is used in a nuclear reactor it contains nuclear fuel, usually in the form of pellets containing enriched UO2. The cladding tube with its content thus constitutes a fuel rod. Because of the very particular environment in which cladding tubes are used, different requirements must be fulfilled.
There are mainly two kinds of modern light water reactors: boiling water reactors (BWR) and pressure water reactors (PWR). In these kinds of reactors different conditions exist, which call for different requirements on the parts that are included in the reactors. In a PWR, the fuel rods are cooled mainly by water that is in a liquid phase under a high pressure. In a BWR, the pressure is lower and the water that cools the fuel rods is evaporated such that the fuel rods are surrounded both by water in a liquid phase and in a steam phase. Furthermore, the fuel assemblies have different construction in a BWR and a PWR. In a certain kind of BWR, the fuel rods in a fuel assembly extend the whole way between a top plate and a bottom plate which keep the fuel assembly together. In a PWR, on the other hand, the fuel rods are usually kept in position with the help of spacers and do not reach all the way to the top plate and to the bottom plate.
When a fuel rod is used in a nuclear reactor, it is exposed to neutron radiation. This leads to the fact that the cladding tube tends to grow with time. In certain kinds of BWR, the cladding tube has only a limited possibility to expand in the longitudinal direction. The cladding tube may therefore bend during operation. This can lead to damages. It should therefore be avoided that the cladding tube grows to a larger extent. Modern cladding tubes which are produced in suitable zirconium alloys and which undergo special heat treatments during the production often have a relatively low tendency to grow when they are exposed to neutron radiation. The tendency to grow may be reduced, inter alia, in that the cladding tube during the production undergoes a final recrystallization anneal.
Through a suitable choice of the material for the cladding tube and a suitable method of production, the cladding tube can obtain suitable properties concerning for example hardness and ductility. Since the conditions are different in a BWR and a PWR, the cladding tubes are produced with different properties depending on for which kind of reactor they are made.
In the environment where the cladding tubes are used they are subject to different corrosive attacks. These attacks may come from the outside or from the inside. The attacks from the inside often have their basis in an influence from the nuclear fuel material that is located there, so-called pellet-cladding interaction (PCI). If a crack is formed through the cladding tube (a so-called primary damage), water may penetrate in through the crack and spread along the inside of the tube. This may lead to new corrosive attacks from the inside of the tube, so-called secondary damages. A cladding tube of zirconium may also react with hydrogen such that hydrides are formed in the cladding tube. These hydrides may be formed from the inside of the tube, particularly if a crack has been formed such that water has penetrated into the tube. These hydrides make the tube more fragile and the probability for the formation of cracks increases. Particularly hydrides that extend in a radial direction through the tube constitute an increased risk for crack formation. Such radial hydrides may therefore speed up possible secondary damages and crack formations.
The complicated chemical, mechanical and metallurgical conditions that are the case in a nuclear reactor have lead to the fact that a very large number of suggestions have been proposed for the selection of materials and for the methods of production of cladding tubes. Even small changes in the composition of alloys or production parameters may have a large importance for the properties of the cladding tube.
Since different conditions are the case on the inside and on the outside of the cladding tube, cladding tubes are sometimes produced with different compositions in different layers. The above mentioned document EP 0 674 800 B1 thus describes the production of a cladding tube which has an outer component that is made of for example any of the frequently occurring alloys Zircaloy 2 and Zircaloy 4. The cladding tube has an inner component—a so-called liner—which according to an embodiment mainly consists of Zr with the alloying elements 0.25% Sn, 310 ppm Fe and 430 ppm O. The cladding tube is produced according to a particular method with carefully selected heat treatments. The cladding tube undergoes a final anneal at 570° C. during 1.5 h, which means a complete re-crystallization anneal (cRXA). The produced cladding tube has been shown to have a good resistance against corrosion even if water happens to penetrate into the inside of the cladding tube.
Another example of a cladding tube is clear from U.S. Pat. No. 4,933,136. This document describes a cladding tube consisting of an outer component of Zircaloy 2 or Zircaloy 4 and an inner component which according to one embodiment mainly consists of Zr with 0.19-0.20 percentage by weight Sn, 0.19 percentage by weight Fe and 615-721 ppm O. The document describes the production of the tube with different rolling steps and heat treatments. As a final anneal three alternatives are described in the document. According to the first alternative, a complete recrystallization (cRXA) occurs in both the outer and the inner component. According to a second alternative, a cRXA occurs in the inner component but only a stress relief anneal (SRA), i.e. no noticeable recrystallization, in the outer component. According to a third alternative, a partial recrystallization (pRXA) occurs in the inner component and an SRA in the outer component.
For cladding tubes which are constructed with two layers and which are intended to be used in a BWR, usually a final anneal is carried out which leads to a cRXA in both the layers. Thereby, a good resistance against damages caused by PCI can be achieved at the same time as the cladding tube has a good ductility and also obtains a structure that counteracts growth caused by neutron radiation.